U.S. patent application number 12/018762 was filed with the patent office on 2009-07-23 for compact thermoacoustic array energy converter.
Invention is credited to Young S. Kwon, Orest G. Symko.
Application Number | 20090184604 12/018762 |
Document ID | / |
Family ID | 40875910 |
Filed Date | 2009-07-23 |
United States Patent
Application |
20090184604 |
Kind Code |
A1 |
Symko; Orest G. ; et
al. |
July 23, 2009 |
COMPACT THERMOACOUSTIC ARRAY ENERGY CONVERTER
Abstract
A thermoacoustic array energy converter consists of heat driven
thermoacoustic prime movers in parallel coupled by means of an
acoustic cavity to a piezoelectric electrical generator whose
output is rectified and fed to an energy storage element. The prime
movers convert heat to sound in a resonator. The sound form a
phase-locked array is converted to electricity by means of the
piezoelectric element. The generated electric energy is converted
to DC by means of a rectifier set and it is then stored in a
battery or supercapacitor. The generated electric energy can also
be converted to power line frequency.
Inventors: |
Symko; Orest G.; (Salt Lake
City, UT) ; Kwon; Young S.; (Salt Lake City,
UT) |
Correspondence
Address: |
MORRISS OBRYANT COMPAGNI, P.C.
734 EAST 200 SOUTH
SALT LAKE CITY
UT
84102
US
|
Family ID: |
40875910 |
Appl. No.: |
12/018762 |
Filed: |
January 23, 2008 |
Current U.S.
Class: |
310/334 ;
320/101; 60/517 |
Current CPC
Class: |
F02G 1/043 20130101;
F02G 2243/54 20130101; H02N 2/18 20130101 |
Class at
Publication: |
310/334 ;
320/101; 60/517 |
International
Class: |
H02J 7/32 20060101
H02J007/32; H02N 2/18 20060101 H02N002/18; F02G 1/043 20060101
F02G001/043 |
Goverment Interests
GOVERNMENT RIGHTS
[0001] This invention was made with government support under
N0001403-1-1-0543 awarded by the Office of Naval Research. The
Government has certain rights to this invention.
Claims
1. A thermoacoustic energy converter for converting heat energy to
electricity, comprising: a plurality of resonators, each having a
first end, a second open end, defining a resonator chamber and a
stack disposed within said resonator chamber; an acoustic chamber
coupled to and in fluid communication with each of the second open
ends of the plurality of resonators, a working fluid disposed
within the resonator chambers interior chamber; an
electro-mechanical transducer coupled to the acoustic chamber and
in communication with the working fluid, wherein vibrations from
the working fluid on the electro-mechanical transducer actuate the
electro-mechanical transducer to generate electricity; and an
acoustic chamber disposed at the second end of the resonator,
wherein the acoustic chamber reflects and amplifies at least a
portion of a sound wave back towards the first end of each of said
resonators; whereby the acoustic chamber reflects and amplifies at
least a portion of a sound wave generated by said plurality of
resonators back toward a first end of each of said plurality of
resonators.
2. The thermoacoustic energy converter of claim 1, wherein said
stack comprises a first heat exchanger disposed adjacent to a first
side of said stack and thermally coupled to a hot side of said
resonator, a second heat exchanger disposed adjacent to a second
side of said stack and thermally coupled to a cold side of said
resonator, said cold side of said resonator being thermally coupled
to said acoustic chamber.
3. The thermoacoustic energy converter of claim 1, further
comprising a thermal coupling mechanism coupled to each resonator
to transfer heat energy from the thermal coupling mechanism to the
plurality of heat exchangers for creating at least one standing
wave within each resonator.
4. The thermoacoustic energy converter of claim 3 wherein a face of
the electro-mechanical transducer is generally disposed parallel to
and is aligned generally coaxially with the stacks of the plurality
of resonators.
5. The thermoacoustic energy converter of claim 1, wherein the
electro-mechanical transducer is disposed at an end of the acoustic
chamber and in fluid communication with the working fluid disposed
within the acoustic chamber.
6. The thermoacoustic energy converter of claim 1 wherein each
resonator has a generally cylindrical resonator chamber, a tapered
cylindrical resonator chamber, or a Helmholtz-like resonator
chamber.
7. The thermoacoustic energy converter of claim 1 wherein said
stack is comprised of a random fiber stack material selected from
the group of materials comprised of cotton wool and glass wool.
8. The thermoacoustic energy converter of claim 1 wherein the
working fluid is selected from the group of gases comprising air,
an inert gas, and a mixture of inert gases.
9. The thermoacoustic energy converter of claim 1, wherein the
electro-mechanical transducer is comprised of a piezoelectric
element that is capable of being actuated by sound at frequencies
greater than 2000 Hz, and generating electricity therefrom.
10. The thermoacoustic energy converter of claim 9 wherein said
piezoelectric element is capable of being actuated by sound at
ultrasonic frequencies, and generating electricity therefrom.
11. The thermoacoustic energy converter of claim 1 wherein the heat
exchangers are comprised of thermally conductive metal mesh.
12. A thermoacoustic energy generator, comprising: a heat coupling
element; a plurality of thermoacoustic resonators coupled to said
heat coupling element, each of said resonators capable of
generating a standing wave therein when subjected to a heat energy
from said heat coupling element and having a closed first end and a
second open end; an acoustic chamber having a first end coupled to
and in fluid communication with each of the second open ends of the
plurality of resonators, the acoustic chamber having a volume that
is greater than the resonator chamber of one of said plurality of
resonators; a working fluid disposed within the resonator chambers
and the acoustic chamber; a piezoelectric transducer coupled to a
second end of the acoustic chamber and in communication with the
working fluid, wherein vibrations from the working fluid on the
piezoelectric transducer actuate the piezoelectric transducer to
generate electricity.
13. The thermoacoustic energy generator of claim 12, wherein each
of said plurality of thermoacoustic resonators comprise a first
heat exchanger disposed adjacent to a first side of a stack and
thermally coupled to a hot side of said resonator, a second heat
exchanger disposed adjacent to a second side of said stack and
thermally coupled to a cold side of said resonator, said cold side
of said resonator being thermally coupled to said acoustic
chamber.
14. The thermoacoustic energy generator of claim 12, wherein a face
of the piezoelectric transducer is generally disposed parallel to
and is generally aligned coaxially with the stacks of the plurality
of resonators.
15. The thermoacoustic energy generator of claim 12, wherein the
piezoelectric transducer is disposed at an end of the acoustic
chamber and in fluid communication with the working fluid disposed
within the acoustic chamber.
16. The thermoacoustic energy generator of claim 12, wherein each
resonator has a generally cylindrical resonator chamber, a tapered
cylindrical resonator chamber, or a Helmholtz-like resonator
chamber.
17. The thermoacoustic energy generator of claim 13, wherein said
stack is comprised of a random fiber stack material selected from
the group of materials comprised of cotton wool and glass wool.
18. The thermoacoustic energy generator of claim 12, wherein the
working fluid is selected from the group of gases comprising air,
an inert gas, and a mixture of inert gases.
19. The thermoacoustic energy generator of claim 12, wherein the
piezoelectric transducer is capable of being actuated by sound at
frequencies greater than 2000 Hz, and generating electricity
therefrom.
20. The thermoacoustic energy generator of claim 19, wherein said
piezoelectric transducer is capable of being actuated by sound at
ultrasonic frequencies, and generating electricity therefrom.
Description
TECHNICAL FIELD
[0002] The present invention relates generally to systems and
methods for the generation of electricity in the conversion of heat
to sound in a resonator. More particularly, the present invention
relates to systems and methods for directly generating electricity
in the conversion of heat to sound in a miniaturized device which
is compact light, and capable of handling high power densities.
BACKGROUND
[0003] The concept of converting heat to sound has been known for
over two hundred years. For example, in the "singing pipe," heat is
applied to a closed end of a resonant tube having a metal mesh
within the tube which has a "hot" end near the heated end of the
resonant tube and a "cold" end further from the heat source. The
terms "hot" and "cold" refer to their relative temperatures with
respect to each other. The "hot" end could be at room temperature
with the important parameter not being the actual temperature, but
the temperature gradient.
[0004] An acoustical standing wave set up in the resonator tube
forces a working fluid (e.g., a gas) within the resonator to
undergo a cycle of compression, heating, expansion, and cooling. In
this case, thermal energy is converted into acoustical energy and
it maintains the standing waves.
[0005] The work of converting heat to sound has been moved forward
through the development of thermoacoustical refrigerators, as
disclosed in U.S. Pat. No. 6,574,968, entitled HIGH FREQUENCY
THERMOACOUSTIC REFRIGERATOR, which is incorporated herein by
reference. Essentially, the conversion of heat to electricity by
the present invention can be thought of as the opposite process
performed by the thermoacoustic refrigerator. Thus, instead of
applying energy to a piezoelectric element to thereby cool a
device, energy is being taken and converted from a heat source
itself.
[0006] Early attempts to create a thermoacoustic energy converter
have failed for various reasons. For example, the process was
performed in prior art devices operating at around 100 Hz which
would convert the low frequency sound to electricity. However, the
process was abandoned by those skilled in the art because of the
very low efficiency of the energy conversion process at low
frequencies.
[0007] One prior art process for direct conversion of heat to
electricity utilizes a permanent magnet and a moving coil. This
process is costly because of the magnet. It is also bulky and heavy
and the efficiency decreases as the frequency of the device
increases, making high frequency operation impractical. The device
itself can also cause magnetic interference with nearby
magnetically sensitive devices, precluding use in certain
environments.
[0008] In order to make a thermoacoustic energy conversion process
practical, it may be desirable to operate the device at high
frequencies. High frequencies can result in more efficient
operation of an electro-mechanical transducer, such as a
piezoelectric element that is to be used in the present invention
for the conversion of sound energy to electricity.
[0009] Another advantage of operation at high frequencies comes
from a comparison with prior art thermoacoustic devices that are
relatively large compared to semiconductor devices and biological
samples. Thus, it would be another advantage to make the
thermoacoustic energy converter small enough to be operable with
such devices and samples.
[0010] Attempts to address the shortcomings of the prior art have
resulted in devices, such as that disclosed in the published
International Patent Application entitled High Frequency
Thermoacoustic Energy Converter, International Publication Number
WO 03/049491, which is incorporated by reference herein in its
entirety. Such devices addressed the problems with other prior art
devices by using a resonator that also functions as a housing for
an electro-mechanical transducer, a stack formed from random fibers
comprised of a material having poor thermal conductivity and a pair
of heat exchangers comprised of a material having good thermal
conductivity positioned on opposite sides of the stack. However,
positive feedback across the system was less than desired and
electrical generation was thus reduced.
[0011] Electronic devices and machinery produce waste heat which
limits their performance and efficiency. Thermal management of such
heat and its conversion to electrical power would raise their
output and at the same time provide an important source of
renewable energy. Achieving such goals with simple, efficient and
high power density devices would assist in providing a solution to
current energy problems. The effectiveness of such an approach will
be determined by the nature of the devices, on their ability to
cope with a wide range of heat inputs from waste heat, and on their
impact on the environment.
[0012] Problems that need to be solved deal with device interfacing
to the source of waste heat and device scaling to a wide range of
heat sources including compact electronics. Moreover with
escalating power levels in waste heat, it is important for the
devices to be high power density units in order to cope with high
power level demands. Thus, there is an ever-increasing need for
more energy to be reduced by providing renewable energy from waste
heat. As there is an abundance of such waste heat, an efficient
technology is needed for converting the waste heat to electricity.
Such a technology would be capable of interfacing with sources of
waste heat, would have an extended life and would be relatively
inexpensive to manufacture and implement. A system or method
capable of addressing these issues and of handling the dual
function of energy conversion and thermal management for a wide
range of applications would be an improvement in the art.
SUMMARY
[0013] A thermoacoustic device includes a compact resonant system
which converts heat to sound in a resonator and which transforms
the sound directly to electricity at levels which can be used to
power other electrical systems. Heat applied to one end of the
resonator sets up a sound wave which is coupled by means of a
cavity to a sound-to-electricity converter, such as a piezoelectric
monomorph energy converter. The electrical energy at audio or
ultrasonic frequencies is rectified for storage through electrical
circuitry and components. In order to increase the energy level and
thus the production of electricity derived from a heat source, a
plurality of thermoacoustic devices are used. The thermoacoustic
devices are phase-locked by the cavity and are coupled to a single
sound-to-energy converter. The cavity reduces temperature
difference for the onset of oscillations in each thermoacoustic
device and to phase-lock them for maximum energy output. A low
onset diode full-wave rectifier set is employed with the
piezoelectric device to provide a DC output for energy storage on a
battery or a supercapacitor.
[0014] Injected heat generates sound in each acoustic resonator of
the array of thermoacoustic devices, which is then coupled to a
sound chamber. The sound in the chamber is converted to electricity
by means of a sound-to-electricity converter (i.e., the energy
converter). The chamber allows for sustained acoustic oscillations
in the resonator with additional positive feedback provided by the
cavity coupled to each resonator. It also phase-locks each
thermoacoustic device to provide coherence between each of the
acoustic devices for maximum energy output. The
sound-to-electricity converter directly coupled to the chamber to
generate maximum electrical power from heat-generated sound. Such
devices, while useful as energy converters, may also provide
thermal management in a variety of large and small systems that
produce waste heat in operation.
[0015] In accordance with one aspect of the present invention, heat
from a heat source or waste heat is coupled by a thermal conductor
to the hot side of each thermoacoustic device. Each thermoacoustic
device is coupled to an in fluid communication with a sound
chamber.
[0016] In accordance with another aspect of the present invention,
the energy converter is positioned at a distal end of the sound
chamber.
[0017] In accordance with yet another aspect of the present
invention, the energy converter is in resonance with the acoustic
devices, leading to maximum electrical energy output by the energy
converter.
[0018] In accordance with still another aspect of the present
invention, a single energy converter in the form of a piezoelectric
element is used to collect acoustic energy form multiple acoustic
units, thus providing considerable reduction the volume of the
energy conversion system of the present invention.
[0019] In accordance with another aspect of the present invention,
audio or ultrasonic frequency energy is changed to DC electricity
by means of an efficient rectifier and stored in a battery or a
supercapacitor.
[0020] In accordance with yet another aspect of the present
invention, the storage of electrical energy in a supercapacitor
provides a system with long cycle lives and overall superior
performance to energy storage in a battery.
[0021] In accordance with still another aspect of the present
invention, the energy conversion system of the present invention is
provided in a portable device.
[0022] In accordance with another aspect of the present invention,
the plurality of thermoacoustic units form a large array operating
in the ultrasonic range.
[0023] Methods of utilizing such devices, particularly in arrays to
capture waste heat from electronic devices or machinery are also
included in the present invention, as are methods of creating such
devices.
DESCRIPTION OF THE DRAWINGS
[0024] It will be appreciated by those of ordinary skill in the art
that the various drawings are for illustrative purposes only. The
nature of the present invention, as well as other embodiments of
the present invention, may be more clearly understood by reference
to the following detailed description of the invention, to the
appended claims, to the appendix attached hereto, and to the
several drawings.
[0025] FIG. 1 is a perspective top view of the structural
components and a schematic representation of the electrical
circuitry of one illustrative embodiment of a thermoacoustic energy
converter in accordance with the principles of the present
invention.
[0026] FIG. 2 is a schematic a side plan view of the structural
components and a schematic representation of the electrical
circuitry of another illustrative embodiment of a thermoacoustic
energy converter in accordance with the principles of the present
invention.
[0027] FIG. 3 is a schematic, cross-sectional, side plan view of
another embodiment of a thermoacoustic energy converter in
accordance with the principles of the present invention.
DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS
[0028] The present invention relates to systems and methods related
to thermoacoustic devices. It will be appreciated by those skilled
in the art that the embodiments herein described, while
illustrating certain embodiments, are not intended to so limit the
invention or the scope of the appended claims. Those skilled in the
art will also understand that various combinations or modifications
of the embodiments presented herein can be made without departing
from the scope of the invention. All such alternate embodiments are
within the scope of the present invention. Similarly, while the
drawings depict illustrative embodiments of the devices and
components in accordance with the present invention and illustrate
the principles upon which the device is based, they are only
illustrative and any modification of the invented features
presented here are to be considered within the scope of this
invention.
[0029] In view of the demands for thermal management and for energy
conversion from heat to electricity, an efficient and compact unit,
based on thermaoacoustics, was developed. In such a device, where
heat produces sound and in the same unit the sound is directly
converted to electricity, a resonant acoustic system, heat
exchangers, a stack of fibrous material, an acoustic cavity, and an
electrical generator are provided.
[0030] In a typical arrangement, the stack may be located about
halfway in the resonator with the heat exchangers at each end of
the stack. Heat is injected to the closed end of the resonator,
near or at the hot heat exchanger. The other end of the resonator
is maintained at a fixed lower temperature by means of heat sinks
or heat fins. Such configuration creates a temperature difference
and hence a temperature gradient along the stack. Above a threshold
temperature difference acoustic oscillations are set up in the
resonator. The frequency of heat-generated acoustic oscillations is
determined by the resonator and its size. By attaching an acoustic
cavity to the open end of the resonator, the threshold of onset for
oscillations is reduced substantially. The sound to electricity
generator may be located inside the device at the closed end of the
cavity. The electrical generator may be a piezoelectric element in
the monomorph or bimorph configuration.
[0031] In one embodiment of the present invention, the closed end
of the cavity may contain the electric generator. In other
embodiments, it may be located at the closed end of the resonator.
However, at such location it could be affected by heat injected
into the device.
[0032] In accordance with another aspect of the present invention,
the heat source energy can be coupled to the device by thermal
conduction or it can be a source of energy such as a flame with
directed heating or radio-isotope.
[0033] In accordance with another aspect of the present invention,
the thermal mass of the heated side of the resonator should be much
less than the thermal mass of the fixed low temperature end of the
resonator.
[0034] In accordance with another aspect of the present invention,
an acoustic cavity is attached to a resonator in order to lower the
temperature difference across the stack for onset of
oscillations.
[0035] In accordance with another aspect of the present invention,
the internal diameter of the resonator is kept large even though
devices are miniaturized. In that case initiation of oscillation is
promoted by acoustic cavity. A large resonator diameter leads to
large acoustic power density. In accordance with another aspect of
the present invention, miniaturized devices lead to array
configuration.
[0036] FIG. 1 shows a thermoacoustic electrical generation system
10 where heat energy 12, represented by a candle flame, is
converted to sound and also directly to electricity in a compact
unit in accordance with the present invention. The heat 12 is
converted to electricity which is then stored in a supercapacitor
14. The system 10 can be used to generate electricity and/or to
thermally manage a device or machine which produces excess heat by
converting the heat to sound. Such a system 10 can be used in large
or small systems where waste heat 12 is generated and it can also
be used as a source of electrical power. The energy converter 10 is
comprised of a plurality of thermoacoustic devices 21-25 that are
coupled at a first end to a heat conductive element 26, such as a
metal plate, and coupled at a second end to a sound chamber 28.
[0037] Electricity is generated by a sound-to-energy converter 16,
which may be in the form of an electro-mechanical transducer such
as a piezoelectric transducer, from the sound created inside a
sound chamber 28. The sound chamber 28 defines an interior acoustic
chamber 29 within which sound produced by the acoustic devices
21-25 forms and resonates. The acoustic chamber is generally
cylindrical in shape and has an end opposite the transducer 16 that
is in fluid communication with each open end of each resonating
acoustic device 21-25. Thus, the acoustic chamber 29 is larger than
each of the individual interior chambers of the acoustic devices
21-25. The heat energy 12 is injected into the system by conduction
or direct contact to the upper plate 26 of the energy converter 10.
A heat sink 30 is provided to help maintain the cold side of the
resonators 21-25 at ambient temperature. In order to convert the
electricity generated from the energy converter 16 to useful
energy, a transformer 32 is electrically coupled to the energy
converter 16. A diode bridge or bridge rectifier 34 is provided to
convert an alternating current input into a direct current output.
The electricity may be stored in a supercapacitor 14 to power a
light 36 or some other electrical appliance that is operated by a
switch 38.
[0038] Referring now to FIG. 2, there is illustrated another
embodiment of a thermoacoustic energy converting system, generally
indicated at 100, in accordance with the principles of the present
invention. The system is configured similarly to the system 10
previously described, but is coupled to a battery 101 for storage
of electrical energy generated by the system 100. As further
illustrate in FIG. 2, each thermoacoustic device 102-104 is
comprised of a coupling structure 106 for mounting the first end
108 of the resonator to a structure 110 conducting heat from a heat
source 112. Thus, the first end 108 forms a hot side of each
resonator. Each resonator 102-104 also includes a cold side 114
that is coupled to an acoustic chamber structure 116. The acoustic
chamber 116 is maintained at ambient temperature with a plurality
of heat sinks 117. The open ends 118 of each resonator 102-104 are
in fluid communication with the acoustic cavity 120. Thus, sound
waves emanating from the open end 118 of each resonator 102-104
will enter the acoustic cavity 120 causing actuation of the
piezoelectric driver 122 to produce electricity.
[0039] As illustrated in FIG. 37 each thermoacoustic resonator 151,
152 and 153 of the energy converter 150 according to the present
invention generates a standing wave 154, 155 and 156, respectively,
when the hot side 157, 158 and 159 of each resonator 151, 152 and
153, respectively, is heated by a source 160 of heat. The heat
source 160 may be thermally coupled to other structural components
of other systems (not shown) that generate heat, such as electrical
systems, integrated circuits, microprocessors or any other
components or systems that create heat. The heat is conducted
through the heat source 160, in this case a metal plate 160',
through the thermal coupling mechanisms 161, 162 and 163, comprised
of cylindrical members attached to the plate 160' with threaded
fasteners such as threaded fastener 164, and into each the proximal
ends of each resonator 151, 152 and 153.
[0040] The standing waves 154, 155 and 156 generated in each
resonator 151, 152 and 153 may be half waves or quarter waves
depending on the length L of the resonator chamber 151', 152' and
153' of each resonator. Each resonator 151, 152 and 153 is
comprised of a stack 165, 166 and 167. Each stack, such as stack
165, is comprised of a hot heat exchanger 168, a cold heat
exchanger 170 and a stack material 172 disposed between and in
thermal contact with each heat exchanger 168 and 170. The stack
material may comprise cotton wool, glass wool, steel wool, aerogel,
other fibrous materials, a series of perforated plastic plates or a
plurality of longitudinally aligned nanotubes. The optimum spacing
between the randomly arranged fibers in the stack 165, when a
fibrous stack is utilized, may be determined by the thermal
penetration depth for the working fluid or gas 190, such that the
acoustic field can interact thermally with each element of the
stack 165. Typical volume-filling factors for stack 165 may be from
about 1% to about 2%. The stack 165 is positioned within the
resonator chamber 151' at a distance from the distal end 175 where
the greatest pressure gradient across the stack 165 is achieved.
The hot heat exchanger 168 is thermally coupled to the hot end 157
of the resonator 151 and thermally isolated from the cold heat
exchanger 170 and cold end 174. The cold heat exchanger 170 is
thermally anchored to a fixed temperature, usually ambient
temperature. Both heat exchangers 168 and 170 may be formed as a
thin screen which is effectively acoustically transparent, but that
maintains a fixed temperature at each end of the stack 165. The
heat exchangers 168 and 170 may be formed from laser patterned
copper or aluminum. Thus the thermoacoustic energy converter 150 is
comprised of a plurality of resonators 151, 152 and 153, each
having a first closed end 157', 158' and 159', a second open end
175, 177 and 179 and defining a resonator chamber 151', 152' and
153'. The stack 165 is disposed within the resonator chambers 151',
152' and 153', The acoustic chamber 178 is coupled to and in fluid
communication with each of the second open ends 175, 177 and 179 of
the plurality of resonators 151, 152 and 153. A working fluid 190
is disposed within the resonator chambers 151', 152' and 153' as
well as the interior acoustic chamber 178. A electro-mechanical
transducer 182, in this case a piezoelectric driver, is coupled to
the acoustic chamber 178 and has a face 182' in fluid communication
with the working fluid 190 such that vibrations imparted by the
resonators 151, 152 and 153 to the working fluid 190 cause the
electro-mechanical transducer 182 to generate electricity. The
stack 165 has a first side 165' and a second side 165'' with the
stack 165 being disposed within the resonator 151. A temperature
gradient is formed between the first and second sides 165' and
165''. The heat exchanger 168 is positioned adjacent the first side
165' of the stack 165 and the heat exchanger 170 is disposed
adjacent to the second side 165'' of the stack 165. The acoustic
cavity or chamber 178 is in fluid communication with the second
open ends 175, 177 and 179 of the resonators 151, 152 and 153. The
acoustic cavity 178 reflects and amplifies at least a portion of
the sound waves 176 back towards the first ends 157' 158' and 159'
of the resonators 157, 158 and 159 such that the acoustic chamber
178 reflects and amplifies at least a portion of the sound waves
176 generated by the resonators 151, 152 and 153 back toward the
first ends 157', 158' and 159' of each of the resonators 151, 152
and 153.
[0041] The resulting temperature gradient across the stack 172
creates a standing acoustic wave 154 within the resonator chamber
151'. The working fluid 190, such as air, helium, argon,
combinations thereon or other known gases is disposed within the
resonator and acoustic chambers 151', 152', 153' and 178. The
working fluid 190 allows the standing waves 154, 155 and 156 to
form.
[0042] Thus, each thermoacoustic device 151, 152 and 153 is
comprised of two sections defining a hot side and a cold side. Each
section of the thermoacoustic device has a heat exchanger thermally
anchored to its end adjacent the stack. The stack 165, formed from
a porous, high surface area material, is thermally anchored to each
heat exchanger by abutting therewith. The stack material is
configured to be able to maintain a temperature gradient. A
temperature gradient along the stack is achieved and maintained by
injecting heat to the hot side of the resonators and maintaining
the cold side of the resonator at a fixed temperature, such as room
temperature or other ambient temperature. A thermal heat sink 186
is provided to maintain the cold side at ambient temperature.
[0043] Because the distal end 175 of the resonator chamber 151' is
open, sound waves 176 emanating from the distal end 175 can enter
an acoustic cavity or chamber 178. The sound waves 176 from each
resonator 151, 152 and 153 combine within the chamber 178 to create
phase-locked sound waves 180 that are directed to and impinge upon
the piezoelectric transducer 182. When the transducer 182 is
actuated, electrical current is generated and sent through the
electrical leads 183 and 184 of the transducer 182 to an electrical
circuit as previously described herein. The transducer 182 is
generally disposed parallel to and aligned generally coaxially with
the stacks of the plurality of resonators
[0044] The energy converter 150 is configured to operate from the
mid-audio frequency range to the ultrasonic range (e.g., from about
2 to 2.6 kHz to about 24 kHz and higher frequencies). In these
frequency ranges, advantage is taken of the high sensitivity of
piezoelectric devices and their compactness. An electric type of
transducer, such as a piezoelectric device, for sound to
electricity conversion is superior to an electromagnetic type when
operated at high frequencies and when compactness is an issue, as
in the miniaturization of devices. Since the thermoacoustic devices
are resonant systems, their size determine the resonant frequency,
and hence by miniaturizing them, the operating frequency is raised
accordingly. The choice of device size is determined by the
application and by how much power needs to be converted. Units
consisting of arrays offer the possibility of dealing with large
power levels which maintain compactness and offer lightweight
systems.
[0045] Performance of the system of the present invention is
determined by operating conditions, mainly the temperature
difference imposed by the source of heat driving the system. The
amount of heat and the resulting temperature difference will
determine the power output, its efficiency and onset for
oscillation. The larger the heat input, the higher the sound level
will be in the resonator resulting in greater electrical
generation.
[0046] In an array of thermoacoustic devices according to the
present invention, the thermoacoustic devices should be
"phase-locked." That is, because the operating frequency of each
individual thermoacoustic device may be slightly different, their
resulting phases will also be slightly different, depending on the
initial conditions for onset of oscillation. In a self-sustained
oscillator, the initial phase is usually arbitrary. Thus, an array
phase-locking allows the system to achieve maximum power output.
Phase-locking of the individual thermoacoustic devices is achieved
by the addition of the acoustic chamber of the present invention
which provides coupling for in-phase motion of all the acoustic
units. When such phase-locking is achieved, the power output
depends directly on the number of thermoacoustic devices in the
array and maximum power output is achieved.
[0047] The energy converter of the present invention is a device
which has essentially no moving parts (other than the gas in the
resonator and sound chamber and the flexing of the piezoelectric
device). The gas may be comprised of air, but may also be helium,
gas mixtures or argon and helium, or other gases known in the art.
In addition, the working gas may be pressurized for higher power
density.
[0048] By operating in the mid-audio and low ultrasonic frequency
ranges, the thermoacoustic devices are relatively small and can be
easily pressurized to high pressure levels. In accordance with the
principles of the present invention, the thermoacoustic devices can
be pressurized to pressures such as 100 atmospheres and higher
without problems related to strength of materials.
[0049] The resonator 151 determines the frequency of the
thermoacoustic engine. It does this by setting up a standing wave
154 from acoustic pulses generated by the temperature gradient
along the stack 165. The resonator 154 provides positive feedback
at the stack 165 which sustains the acoustic oscillations. The
resonator 151 may be of a one quarter, one half or other wavelength
type. In the case of a one quarter wavelength resonator, the cold
end is open. Because there is a difference in impedance at the open
end between the resonator 151 and the acoustic chamber 178, the
acoustic wave is reflected back into the resonator chamber 151',
thus setting up the standing wave 154. Quantatively, the standing
wave 154 is described by the standing wave ratio. This may be
reduced as the diameter of the resonator is increased relative to
the length leading to a larger amount of traveling component which
is radiated out. A large resonator diameter may be used to provide
a large output since the level of generated sound depends on the
cross-sectional area of the stack 165. A high standing wave ratio
may favor a lower temperature difference for the onset of
oscillations because more positive feedback is provided by the
reflected wave. Hence, a wide, short resonator will require a large
temperature difference across the stack 165 for onset of
oscillation unless more gain is provided for positive feedback. The
resonator 151 is essentially a storage element where acoustic
energy is built up for providing the positive feedback and for
generating the sound which will activate the electrical generator
182. It also provides spatial acoustic phasing for the location of
the stack 165 inside the resonator chamber 151' for optimum
performance in sound production. The only "moving part" in the
device of this invention is the working gas (generally depicted by
arrows on either side of the stack 165 in the resonator 151 which
oscillates at the acoustic frequency determined by the
resonator.
[0050] Another reason for a wide resonator is that the stored
acoustic energy is large in comparison with viscous and thermal
losses within a characteristic surface layer inside the resonator.
The acoustic cavity 178 is used to increase the positive feedback
in the system. This is particularly important when the standing
wave ratio in the resonator 151 is low, which occurs when the
quarter wave resonator is wide but short. The cavity 178 acts as a
reflector which can be non-resonant or resonant. The latter case
may lead to the highest increase in positive feedback to the
resonator 151. Ideally the cavity 178 should be on resonance at or
near resonance with the resonator 151. Another advantage of the use
of the acoustic chamber 178 is that its quality factor or "Q" may
be used to enhance the feedback, depending on geometrical factors.
An important consequence of increased positive feedback from the
chamber 178 is a reduction in the critical temperature difference
across the stack 165 for the onset of oscillation. This may prove
especially advantageous compared to prior devices, as only a low
temperature difference may be available for certain applications.
It will be appreciated that the distal end 188 of the chamber 178
is a convenient location for disposing the electrical generator
182, in this exemplary embodiment a piezoelectric driver, which
converts the sound to energy. Depending on the particular
application, the shape of cavity 178 may be cylindrical in
cross-section, have a tapered cylindrical cross-section, be
Helmholtz-like or any other advantageous geometrical shape.
[0051] The heat-to-electricity generating system 150, of the
present invention may incorporate various approaches to inject heat
to the hot heat exchanger 168. A flame or a heating element can be
used as the source of heat. Heat is injected to the system 150 by
direct heating of the hot heat exchanger 168 or by heating the hot
section 157 of the resonator 151 to which the hot heat exchanger
168 is thermally anchored. Heat from other sources, such as waste
heat from a mechanical or electronic device, can be injected to the
hot heat exchanger 168 by metallic thermal conduction, as depicted
in FIG. 1. Similarly, an appropriate radioisotope containing
element may be used to inject heat to the hot heat exchanger 168,
again by metallic thermal conduction.
[0052] The electrical generator 182 may be a piezoelectric element
in a monomorph configuration or bimorph configuration. Each
piezoelectric element ("piezo") is a capacitor offering high
impedance for current extraction. Electrical generator 182 may be
tuned to the resonance of the resonator 151 to maximize electric
output. Because the electrical generator 182 is a pressure
sensitive unit, optimal performance may be achieved by positioning
the generator 182 at the location of maximum acoustic pressure,
typically, at the distal end 188 of chamber 178 opposite the
thermoacoustic resonators 151, 152 and 153.
[0053] Electrical power output may be maximized when the electrical
generator 182 is in resonance with the acoustic system 150 device.
The electric voltage obtained may be enlarged by configuring the
generator 182 in a bimorph mode, where two piezos attached to a
metallic membrane are connected in series; such piezos may appear
as a bimetallic strip to provide maximum voltage output when
exposed to sound power.
[0054] A system 150 in accordance with the present invention may be
miniaturized for operation in the frequency range of from about 2
kHz to about 24 kHz. Used in arrays, such systems may be configured
to work in the ultrasonic range at 40 kHz, as for military power
applications. High power densities may be achieved by pressurizing
the working fluid 190. Such miniature thermoacoustic energy
converters may work up to frequencies as high as the ultrasonic
range. Operation with a low threshold in temperature difference for
oscillation may make such systems useful for a variety of
applications.
[0055] Used in arrays, systems 150 of the present invention may be
useful in any number of applications. For example, such systems may
be thermally attached to a waste heat producing electronic or
mechanical device, such as a radar system or high powered
electronic devices. The waste heat will drive the system 150 and be
thereby converted to electricity. In other applications, an array
of systems 150 may be used as a portable source of electric power.
For example, heat from a flame may be used to activate the unit and
make electrical power available for application. Such a system
would be useful in an emergency or battlefield situation. A typical
array may consist of about 100 systems 10 (shown in FIG. 1) or
systems 150 (shown in FIG. 3) may be linked in parallel between a
cold plate thermally anchored to ambient temperature and a hot
plate where heat is injected. It is anticipated that using air at 1
atmosphere, such a structure could provide about 7.5 watts and
about 150 watts at 20 atmospheres. Any desired or advantageous
number of systems 150 in such an array may be phase locked and
share a single cavity 178 with a single generator 182. For example,
all 100 systems may share a single cavity, or multiples of 10, 20,
or 25 units may share a single cavity.
[0056] In order that the present invention be best understood, a
quantitative description of the how devices in accordance with the
present invention will now be presented. Typically, heat flow
between air parcels in the sound field and each element of the
stack takes place across a thermal penetration depth
.delta..sub..kappa. which is determined by the thermal properties
of the gas, and the acoustic frequency. This distance is defined
as:
.delta. k = ( 2 .kappa. .omega. ) 1 / 2 ##EQU00001##
[0057] where .omega. is the acoustic angular frequency and .kappa.
is the fluid's thermal diffusivity. With air at one atmosphere,
.delta..sub..kappa.=44 .mu.m at 5 kHz. This characteristic
dimension .delta..sub..kappa. is a guide for determining the
spacing between the elements of the stack, i.e. they should be
separated about 2.delta..sub..kappa. to 3.delta..sub..kappa. spaces
apart.
[0058] Heat flow along the stack can be written as:
Q . 2 = - 1 4 .PI. .delta. .kappa. T m .beta. p 1 u 1 ( .GAMMA. - 1
) ##EQU00002##
[0059] where .beta. is the fluid coefficient of thermal expansion,
T.sub.m is the mean temperature, p.sub.1 is the amplitude of
acoustic pressure oscillations, u.sub.1 is the corresponding air
particle speed of the sound field, // is the perimeter around each
stack element times the number of such elements and .GAMMA. is the
ratio of temperature gradient along the stack normalized to a
critical temperature gradient. The resultant work flow is given
by:
W . 2 = 1 4 .PI. .delta. .kappa. T m .beta. 2 .omega. .DELTA. xp 1
2 .rho. m c p ( .GAMMA. - 1 ) ##EQU00003##
[0060] where .rho..sub.m is the mean density of the fluid, c.sub.p,
its specific heat at constant pressure, and x is the stack length.
The efficiency of this engine can be written as
.eta. = W 2 Q . 2 ##EQU00004##
[0061] which simplifies to
.eta. = .eta. c .GAMMA. ##EQU00005##
[0062] where .eta..sub.c is the Carnot efficiency,
.DELTA. T T m , ##EQU00006##
and .GAMMA. is the ratio of temperature gradient along the stack to
a critical temperature gradient .gradient.T. Oscillations will
occur when .GAMMA.>1.
[0063] The critical temperature gradient .gradient.T. is the
boundary between heat flowing from the stack to air parcels and
heat flowing from the air parcels to the stack. It is given by:
.gradient. T crit = .gamma. - 1 T m .beta. T m .lamda. - tan ( x
.lamda. - ) ##EQU00007##
[0064] where .gamma.=ratio of isobaric to isochoric specific
heats
[0065] =radian wavelength of sound
[0066] x=position of stack relative to nearest pressure autinode in
resonator
[0067] As mentioned above .gradient.T is important because it
determines the magnitude of .gradient.T: for the onset of
oscillations, and this occurs for .GAMMA.>1.
[0068] Since the prime mover will be used in energy conversion
devices, it is important to estimate the power density of this type
of engine. The power per unit volume can be calculated from the
heat flow divided by the engine volume (it varies inversely with
operating frequency f).
H . 2 V .apprxeq. f 2 T m .beta. .rho. m a 2 M 2 ##EQU00008##
[0069] where a is the speed of sound in the fluid, and M is a kind
of Mach number expressing the degree of nonlinear behavior, and it
is equal to P.sub.1/.rho..sub.m.alpha..sup.2. This equation
demonstrates that large power densities can be achieved at high
operating frequency and at high mean gas pressures. One of the
important features of the present invention is that power densities
can be quite high, of order of watts/cm.sup.3, and higher,
depending on the geometry and working conditions. Since systems in
accordance with the present invention are resonant systems,
miniaturization can lead to high frequency operation and
consequently the device power density may be large. Miniature prime
movers, such as the resonators and arrays of the present invention,
may be fabricated using MEMS (micro-electromechanical systems)
technology.
[0070] While this invention has been described in certain
embodiments, the present invention can be further modified with the
spirit and scope of this disclosure. This application is therefore
intended to cover any variations, uses, or adaptations of the
invention using its general principles. Further, this application
is intended to cover such departures from the present disclosure as
come within known or customary practices in the art to which this
invention pertains.
* * * * *